3.6.4.3 blood water potential

• Ultrafiltration in the Bowman's capsule

  • Ultrafiltration forms glomerulus filtrate
  • High hydrostatic pressure forces water & glucose out through the gaps between capillary endothelial cells and into the lumen of the nephron.
  • Large proteins and blood cells stay in the blood
• Diameter of efferent arteriole is smaller than the diameter of the afferent arteriole. Creates a high hydrostatic pressure in the capillaries of the glomerulus.
• Selective reabsorption in the proximal convoluted tubule:

  Glucose enters epithelial cells by facilitated diffusion through carrier proteins. It then moves into the tissue fluid through active transport, diffusing into the blood capillaries alongside water.
• Adaptations of epithelial cells:

  • Microvilli to increase surface area = faster rate of diffusion
  • Many mitochondria for active transport of glucose into the blood
• Loop of Henle maintaining a gradient of sodium ions down the medulla:

  1. Active transport of Na+ ions out of the ascending limb, decreasing water potential of tissue fluid in the medulla
  2. Water potential becomes lower going down the medulla due to the increasing concentration of Na+
  3. H20 leaves the descending limb by osmosis and re-enters blood
  4. Water cant leave the ascending limb so continues to the distal convolutes tubule and collecting duct
  5. A more dilute filtrate remains
• Loop of Henle:

  • Descending limb - permeable to water, water can leave to enter the blood, Na+ ions cant leave but can enter
  • Ascending limb - impermeable to water, Na+ actively transported out at the top
  • Na+ ions can leave by facilitated diffusion at the bottom of the descending limb
• Distal convoluted tubule and collecting duct:

  1. Water leaves filtrate by osmosis
  2. Permeability of walls to water increased by ADH released from posterior pituitary gland
  3. Urea and other waste products move down the collecting duct, eventually to the ureters and to the bladder
• Osmoregulation: Hypothalamus detects the change in blood water potential. Posterior pituitary gland releases antidiuretic hormone ADH.
• Osmoreceptors in hypothalamus:

  1. Water potential of blood decreases causing water to move out of the cell by osmosis
  2. More frequent nerve impulses sent to posterior pituitary gland causing an increased release of ADH
  3. ADH travels in blood to cells of collecting duct and DCT increasing their permeability to water
  4. More water reabsorbed into the capillaries by osmosis increasing water potential
• Effect of ADH:

  ADH binds to receptor proteins in target cells of DCT and CD causing more protein channels (aquaporins) to be inserted into the c.s.m
• Increased water potential: less ADH = Decrease in permeability

  Decreased water potential: More ADH - increase in permeability